My housemate just got a pair of HD 25-1 II (Adidas). They only had a few hours of burn-in.
I tried them, here's some impressions of HD 25-1 II compared to HM5. The first thing I noticed with the Senny's was a veil, and the bass is muddier. (Hopefully, those two aspects will go away with a lot of burn-in.) I feel the bass is bleeding into the mids/vocals, which sort of makes it sound congested. The bass quantity is about the same in both of them (maybe a tad more in the Senny's).
Next, I noticed the harshness of certain songs when using HD 25-1 II, which was also apparent when I had the Creative Aurvana Live!. HM5 's treble is very smooth instead with no hint of sibilance or harshness. The frequency response seems to agree with my assessment; there's a peak at around 8-10kHz according to innerfidelity's graph for HD 25-1 II. This slight harshness doesn't really bother me at all though. Only with the help of HD 25-1 II, I have realised how airy HM5 is. (I guess some people could say they are hollow, I say airy and 3D) Thus, HM5's soundstage is notably bigger.
I love the light weight of such portable headphones, without much sacrifice in SQ. I admit HD 25-1 II is the more comfortable of the two and long hours of listening would be easier on my ears.
He got the Adidas for AUD$150 (very good price I think); I guess I got a better deal on the HM5s. Not as portable nevertheless. On a side note, he aslo showed me his vibrating cheapo ($5, big sale) headphones, he says he uses those for movies. The vibrating part is certainly interesting, yet distracting at the same time. A lot of distortion and muddy as hell.